Low-voltage lighting, such as electronic low-voltage (ELV) and magnetic low-voltage (MLV) lighting, is becoming very popular. Low-voltage lamps allow for excellent, precise sources of illumination, extended lamp life, higher efficiencies than incandescent lamps, and unique lighting fixtures, such as track lighting. To power an electronic low-voltage lamp, an ELV transformer is required to reduce a line voltage (typically 120 VAC or 240 VAC) to a low-voltage level (such as 12 volts or 24 volts) to power the ELV lamp.
Many prior art two-wire dimmers exist for control of ELV lighting loads. A conventional two-wire dimmer has two connections: a “hot” connection to an alternating-current (AC) power supply and a “dimmed hot” connection to the lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus control the intensity of the light. The semiconductor switches are typically coupled between the hot and dimmed hot connections of the dimmer.
Since an ELV transformer is normally characterized by a large capacitance across the primary winding, the ELV lighting load is typically dimmed using reverse phase-control dimming (often called “trailing-edge” dimming), in which the dimmer includes two FETs in anti-serial connection. One FET conducts during the first, positive half-cycle of the AC waveform and the other FET conducts during the second, negative half-cycle of the AC waveform. The FETs are alternately turned on at the beginning of each half-cycle of the AC power supply and then turned off at some time during the half-cycle depending upon the desired intensity of the lamp. To execute reverse phase-control dimming, many ELV dimmers include a microprocessor to control the switching of the FETs.
In order to provide a direct-current (DC) voltage to power the microprocessor and other low-voltage circuitry, the dimmer includes a power supply, such as a cat-ear power supply. A cat-ear power supply draws current only near the zero-crossings of the AC waveforms and derives its name from the shape of the waveform of the current that it draws from the AC supply. The power supply must draw current through the connected ELV lighting load. The FETs must both be turned off (non-conducting) at the times when the power supply is charging. So, the FETs cannot be turned on for the entire length of a half-cycle, even when the maximum voltage across the load is desired.
To ensure that the power supply is able to draw enough current to maintain its output voltage at all times, the FETs are turned off at the end of each half-cycle for at least a minimum off-time. The proper operation of the ELV dimmer is constrained by a number of worst-case operating conditions, such as high current draw by the low-voltage circuitry, worst-case line voltage input (i.e. when the AC power supply voltage is lower than normal), and worst-case load conditions (such as the number and the wattage of the lamps, the types of ELV transformers, and variations in the operating characteristics of the ELV transformers). By considering these worst-case conditions, the minimum off-time is determined by calculating the off-time that will guarantee that the power supply will charge fully for even the worst-case conditions. The resulting off-time generally ends up being a large portion of each half-cycle and constrains the maximum light level of the attached load.
However, the worst-case condition is not normally encountered in practice, and under typical conditions, the FETs could normally be turned off for a shorter amount of time at the end of each half-cycle, thus conducting current to the load for a greater amount of time resulting in a higher intensity of the load that is closer to the intensity achieved when only a standard wall switch is connected in series with the load. Prior art dimmers have held the minimum off-time constant under all conditions, and thus, have suffered from a small dimming range.
Thus, there exists a need for an ELV dimmer that includes a power supply and has an increased dimming range. More specifically, there exists a need for an ELV dimmer that includes a power supply and is able to drive an ELV lighting load above the maximum dimming level of prior art ELV dimmers without compromising the operation of the power supply.